Aquaporin 3 (AQP3), a water/glycerol channel protein, has been found to transport hydrogen peroxide (H₂O₂). Here, we show that H₂O_2, imported via AQP3, is involved in nuclear factor-κB (NF-κB) signalling in keratinocytes and in the pathogenesis of psoriasis. IL-23-mediated induction of psoriasis is reduced in AQP3 knockout mice (AQP3^−/−), and is accompanied by impaired NF-κB activation and intracellular H₂O₂ accumulation. In primary keratinocyte cultures, cellular import of H₂O₂ produced by membrane NADPH oxidase 2 (Nox2) in response to TNF-α is facilitated by AQP3 and required for NF-κB activation by regulation of protein phosphatase 2A. As AQP3 associates with Nox2, we propose that this interplay constitutes H₂O₂-mediated signalling in response to TNF-α stimulation. Collectively, these data indicate that AQP3-facilitated H₂O₂ transport is required for NF-κB activation in keratinocytes in the development of psoriasis.